I have a dim recollection that D.K.M Bell used a .256 (probably a Mannlicher) to shoot either cormorants or pelicans on the wing at well over 100 yards when using up a supply of defective ammunition. He was hitting about one out of two. An Italian visitor supposedly asked where Bell had gotten a shotgun with such a great range.

Birds on the wing with a rifle can be done, given enough skill, eyesight, ammunition, practice and a safe backstop. Not by me, I hasten to add, but there are lots of other things I can't do either that can and have been done by those more able.